Sensors and transmitters that are small and require low levels of power for operation are frequently used for collecting information without being intrusive to their operating environment. For example, a battery powered sensing and transmitting device may be surgically implanted within living tissue to sense and transmit characteristics of the body in which it is implanted.
The lifetime of the battery used within such a sensor often requires additional surgical procedures to periodically replace the battery. Similarly, where a sensor and transmitting device is located within controlled or hazardous environments, it is often a time-consuming and expensive task to periodically replace the battery.
Energy may be collected/harvested from radio frequency (“RF”) waves for use in remote sensors and transmitting devices. One example of this functionality is an RF identification (“RFID”) tag that derives power from an RF wave (e.g., from a transmitting device operating to read the RFID tag) and uses that power to transmit an identification signal. One drawback of this technology is that the RFID tags typically only operate over short distances.
A rectenna is an antenna that includes a rectifier; the rectenna receives RF waves, rectifies the waves and produces direct current (“DC”) power. The DC power produced by the rectenna is dependent on rectenna design, RF wave frequency, RF wave polarization and RF wave power level incident at the rectenna. Typically, the DC power output from the rectenna is conditioned by conditioning electronics before being fed to a powered device (e.g., sensor, microprocessor, transmitter etc.). Where characteristics of the RF wave vary, the DC power output from the rectenna also varies; this affects power conversion efficiency due to loading upon the rectenna by the conditioning electronics which attempts to maintain a constant power output for the powered device.